[amsat-bb] Receiving with an rtl-sdr, desense on transmit

Mac A. Cody maccody at att.net
Sun Apr 30 23:06:17 UTC 2017


I am working on a similar project and started with an RTLSDR device.  I
have since switched to using an SDRPlay RSP2, due to its better input
filter, Digital resolution, and shielding.

One problem with these RTLSDR devices is that they have very broad-banded
input filters.  Consequently, the transmitter signal, although 
attenuated, is
received by the RTLSDR, causing it to desense.  You will need add more
filtering to sufficiently attenuate signals outside of the receiver band
of interest.


Mac / AE5PH

On 04/30/2017 02:57 PM, Koos van den Hout wrote:
> I am trying to go full duplex 'on the cheap' by using an rtl-sdr on the
> receiving side, keeping an ear on the downlink while transmitting.
> Software wise this all seems to work. I use a linux setup, with gpredict to
> track the satellites and calculate doppler shift. The gqrx sdr software can
> act like a radio that gpredict can control so the receiving frequency is
> correct. It controls both gqrx as receiver and my ft-857 as transmitter.
> Hardware wise I bought an rtl-sdr with a small preamp and built the pre-amp
> into a metal case.
> This works up to the level where I received AO-73 and received a weak
> Fox-1A and recorded the pass.
> But when I pressed the transmit button on the FT-857 (tuned to the 70cm
> uplink frequency) the receiver (tuned to the 2m downlink frequency) went
> deaf.
> Some testing with the arrow antenna and transmitting on 70cm shows the
> preamp turns deaf on 2 meter. Even without the preamp but with the internal
> amplifier of the rtl-sdr this happens. Setting the gain to 'automatic'
> causes a huge drop in signal and takes time to recover.
> Complete details, experiences, pictures of my setup and details I left out
> start at
> https://idefix.net/~koos/newsitem.cgi/1490542545
> - has anybody done anything with an rtl-sdr as the receiving side, how do
>    you setup the gain? Is there a solution to fix this?
> - specific to fox-1a: I am reading somewhat different reports of the
>    downlink and uplink frequencies when I search with google. I hope
> http://www.amsat.org/?page_id=4690 is right. I did notice the signal is
> weak even with an amplifier.
> Thanks for any insights and tips,
>                                         Koos PE4KH

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